The present invention relates to the radiography art. It finds particular application in conjunction with diagnostic imaging devices such as computerized tomographic (CT) scanners, single photon emission computed tomography (SPECT), and positron emission tomographic (PET) scanners. However, it is to be appreciated that the present invention may also find application in conjunction with other radiation treatment apparatus and imaging apparatus.
Nuclear cameras generally include radiation detectors which collect radiation transmitted through or emitted from a subject. The detector typically has a scintillation crystal constructed of a large doped sodium iodide crystal. The scintillation crystal is hermetically sealed between a glass plate and an aluminum case. The surface of the scintillation crystal toward the glass plate is polished and optically coupled to the glass plate. The glass plate is in turn, optically coupled with an array of photomultiplier tubes. The opposite surface of the scintillation crystal is treated such that it internally reflects light with a preselected solid angle. The reflective surface is sanded with a preselected sanding pattern and coated with a reflective or semi-reflective material, e.g. Teflon. In this manner, the scintillation crystal is manufactured with a fixed solid angle of reflection.
During a scan, gamma rays enter the detector through a collimator mounted on the face of the detector. The gamma rays strike the scintillation crystal causing the scintillation crystal to scintillate, i.e., emit light photons in response to the gamma radiation. The photons initially travel uniformly in all directions of the scintillation crystal. Some photons travel directly out of the scintillation crystal through the glass plate and into the photomultiplier tubes. Other photons travel back towards the treated reflective surface at the entrance of the scintillation crystal. The reflective surface reflects the photons back towards the photomultiplier tubes. In order to obtain a strong signal for each scintillation event, it is advantageous to have most of the photons generated processed by the photomultiplier tubes. However, it is undesirable to reflect the light back too steeply, and it is also undesirable to reflect the light too diffusely. The angle of reflection is dependant on the size and geometry of the photomultiplier tubes and the thickness of crystal/light pipe (LP).
This reflection technique has some disadvantages. Particularly, the sanding process is a difficult precision process. It is difficult to sand the crystal surface such that the entire surface of the scintillation crystal has the same index of reflectivity.
Other difficulties involve the scintillation crystals light spread function. The spatial resolution of the gamma camera is directly related to the light spread function within the scintillation crystal. The light spread function is dictated by the surface treatment of the crystal's entrance surface. The surface treatment creates a general solid angle of reflection. The reflective properties are determined according to a particular size and geometry of the photomultiplier tube array. Changing the geometry of the photomultiplier tube array or crystal/light pipe thickness requires painstaking modifications to the reflective surface to optimize the light spread function.
A uniform index of reflectivity over the entire surface is not necessarily ideal. The array of photomultiplier tubes on the opposite surface defines a mosaic of high, low, and intermediate light sensitive regions.
The present invention provides a new and improved scintillation crystal assembly and nuclear camera which overcomes the above-referenced problems and others.